1. Field of the Invention
The present invention relates generally to a method for dispensing viscous materials onto a surface through a screen having a plurality of openings therein, and more particulary to a method for screen printing of solder paste onto a surface of a printed circuit board through the screen that is used in producing surface mount technology circuit boards suitable for use in high technology products.
2. Description of the Prior Art
There are various methods by which solder paste can be printed on a printed circuit board. One approach is to use the screen printing apparatus which has a spatula-like squeegee which is arranged to be displaced back and forth along a pair of guide rails and is arranged to be raised and lowered relative to the guide rails. This screen printing apparatus is adapted to be in pressure contact with an upper surface of the screen when it is lowered and placed into a lower position where solder paste is printed onto a surface of a printed circuit board through the medium of the screen. This type of screen printing apparatus is disclosed in the Japanese Patent Gazette No. 1-55597 published on 27th Nov. 1989. In this type of apparatus, a considerable amount of solder paste is manually supplied onto the screen prior to operating the apparatus and the solder paste supplied onto the screen in such a way is printed on the surface of the circuit board by reciprocating the squeegee, while its lower end is pressed against the screen to introduce the solder paste into openings of the screen.
Major disadvantages of this type of screen printing apparatus are that as a considerable amount of solder paste placed on the screen is kept exposed to the air until the apparatus is put into operation, it tends to be solidified due to evaporation of flux contained therein and/or to be oxidized as it contains oxidizable components, such as lead. As the printing of solder paste is performed by reciprocating the squeegee, the solder paste tends to be excessively printed on a surface of the circuit board. As a result, printed films tends to be blurred on the surface of the circuit board and, in addition, a uniform printing effect cannot be obtained in every printing cycle. It is difficult to prevent the solder paste from acquiring of such changes as solidification, oxidization, etc., even if it is frequently kneaded on the screen with manual labour during the period when the apparatus is not in operation. To use such a solidified and/or oxidized solder paste brings about undesirable printing effect. For this reason, it must be removed from the screen and replaced with new solder paste. This means that a considerable amount of solder paste becomes a useless waste to be abandoned in addition, replacement of solder paste not only requires the manual labour, but also requires printing operation to be stopped for a certain period of time. This results in an economic loss and worker exposure to lead which is a health concern.
Another screen printing apparatus is known which comprises a viscous material automatic supply dispenser of the type having a housing of a closed box type in which a chamber for containing the material and a slit-like bottom aperture communicating therewith, having a leading side defined by a front wall and a trailing side defined by a rear wall and being arranged to be normally closed and to be kept open to automatically supply a predetermined amount of the material onto a surface of a board during priting. This type of screen printing apparatus is disclosed in U.S. Pat. No. 2,027,102 and in the Japanese patent application No. H2-419135 publish on Aug. 31, 1992. In this type of known apparatus, the dispenser is provided with a flexible or pliable rear wall which serves as a squeegee to force the material supplied by the dispenser onto an upper surface of a screen into openings in the screen. Another screen printing apparatus which is similar to this type of known apparatus and in which a slit-like bottom aperture in a housing is not arranged to be closed and opened is also known. This type of apparatus is disclosed in U.S. Pat. No. 4,622,239. The apparatus disclosed in this patent is provided with a pair of flexible blades as squeegee blades which are secured to, and depend from a housing on opposite sides of the bottom aperture to contact a screen.
A common feature of these known apparatuses of the type having such a viscous material automatic supply dispenser is that one or two squeegees employed by the dispenser are formed of a urethane rubber or the like. This feature tends to bring about disadvantages when the dispenser is used for screen printing of solder paste onto a surface of a printed circuit board, as will be described later.
Another common feature of these known apparatuses of the type mentioned above is that the dispenser lacks means for adjusting the contacting pressure of a squeegee to be applied against the upper surface of the screen. Lack of such means also brings about disadvantages, as will also be described.
FIG. 13(a) shows the state in which a flexible squeegee N employed by a conventional screen printing apparatus is in pressure contact with a metal screen 1. As shown, the lower end of the squeegee projects partially by deformation into an opening 1A in the screen. As a result, as shown in FIG. 13(b), printed solder paste 4 on a metalized area of a printed circuit board P is deformed into a concave, so that uniformity in the printed paste thickness can not be obtained.
When printing solder paste 4 which normally has a high viscosity with the flexble squeegee N, the squeegee will experience high shear forces, causing the paste to separate into the constituent components (e.g., solder particles and a suspending viscous flux) and a thin film of flux is formed on the screen 1. The flux film left on the screen makes residue of the paste which is detrimental to high quality printing of solder paste onto the printed circuit board P.
The amount of projection of the flexible squeegee N into an opening 1A is larger as the area of each opening is larger, preventing high quality printing.
In the screen printing, printing is repeatedly performed, so that when the contacting pressure of a squeegee applied against a screen is too strong, the screen is caused to be damaged by abrasion. If the viscous material to be printed is solder paste containing solder particles, abrasive friction between the screen and the squeegee will be increased, because the particles are forced to be pushed ahead under pressure. Conversely, if and when said contacting pressure is weak, then some of the flux and some of the solder particles will pass under the squeegee and will be left as solder paste residue. In addition, it will be impossible for the squeegee to introduce a sufficient amount of solder paste into each of the opening in the screen, preventing proper printing. Moreover, the printed solder paste, when depleted of flux, tends to remain in the screen openings and causes the problem of clogging of the openings. It is impossible for the conventional dispensers to adjust the contacting pressure of the squeegee or squeegees to be applied against the screen as occasion demands.